1. Field
The presently disclosed subject matter relates to a vehicle clutch actuator and in particular a hydraulically controlled clutch actuator for a wet-type multiple disc clutch and its associated method.
2. Description of the Related Art
Existing vehicle drive force transfer systems deliver a torque from the engine to the wheels of the vehicle through the vehicle drive train/transmission. In a torque transfer system for on-demand or user actuated four wheel drive modes for a vehicle, power from the engine transmission may be selectively split between the front and rear wheels by incorporating a clutch mechanism in a rear wheel differential.
One example of a user controlled power transmitting device is described in U.S. Pat. No. 5,135,071 to Shibahata et al. Torque from a propeller shaft is transferred to a pair of rear wheel axles through a speed control device and a pair of left and right variable torque transmitting clutches. Each variable torque transmitting clutch is enclosed in a clutch case and contains multiple pairs of inner and outer friction plates or discs which are pressed together by a clutch actuating mechanism. When these pairs of discs are brought together, torque supplied by a common shaft is transferred to the drive axle of a wheel. The clutch actuating mechanism for these plates have sometimes included an electromagnetic actuator that controls a piston with presser members which are used to press the clutch mechanism's inner and outer plate pairs together. However, typically, a hydraulically controlled piston type actuating mechanism has been utilized, such as that described in U.S. Pat. No. 6,848,555 to Takatoshi Sakata et al.
The concept of providing and managing an on-demand type of torque transfer mechanism in which variable torque is provided in a four wheel drive system has also recently been implemented. Such a mechanism is known to provide excellent vehicle stability and control in all types of weather and road conditions. In addition, variable torque four wheel drive systems often minimize the drawbacks of conventional four-wheel drive systems in terms of weight, noise, performance and design capacity limits. U.S. Pat. No. 7,021,445 to Brissenden discloses an on-demand type of variable torque transfer mechanism for incorporation into a vehicle drive train. Unlike conventional on-demand four-wheel drive systems, which often react only to wheel slippage, the conventional variable torque management systems do not wait for wheel slip before beginning activation of semi or total four wheel drive mode. Instead, torque is proactively delivered to the rear wheels whenever the vehicle is accelerating for improved traction and control in both dry and slippery road conditions. When wheel slippage is detected, a variable torque management system can be configured to apportion additional torque to the rear in proportion to the amount of wheel slip. The system can continuously monitor the vehicle's dynamic condition via sensors in the engine, brake and throttle systems, and can adjust front-to-rear torque split for maximum control.
A central, computer-controlled, Power Control Unit (PCU) has been used to determine the right level of torque split (using sophisticated algorithms) for any given moment to provide optimal traction and stability. Torque can be delivered to the rear wheels via an electronic rear differential mechanism that employs a set of electrically controlled wet clutch packs to take up torque from the propeller shaft as the system demands.
Electric clutch actuation for on-demand type clutch mechanisms is currently a very common mode of actuation for the on-demand clutch mechanism. Recently, hydraulic actuation of the clutch has also been attempted. However, there remains a need to advance hydraulic clutch actuation techniques, and to address recognized system limitations. For example, the size, weight, and electrical power consumption requirements along with cooling capacity of current hydraulic clutch mechanisms currently can result in bulky and expensive systems that are sometimes hampered by limited power characteristics, limited reaction time characteristics, limited cooling capacity, limited control, weight considerations, etc.